Jihad is quintessentially a religious doctrine and not withstanding difficulties of conceptualisation, constitutes a foundational principle of both the shari'a (religious law of Islam) and Islamic International law (siyar). Jihad, in all its variants, is designed to further the will and dictates of God Almighty and must be distinguished from worldly struggles such as tribal rivalries and national conflicts or international wars. With its focus on Jihad, this article has the four key objectives. First, since within the historiography of siyar, the Jihad ideology has retained primacy and centrality, this article critically examines the key facets of this ideology. Second, in its historical incarnation, Jihad obligations were placed upon the whole of the Muslim community as fard kifaya (collective obligation). However, in more recent times the doctrine has been appropriated almost exclusively by terrorist organisations, who have insisted that Jihad is in fact fard 'ayn (individual obligation) to be pursued aggressively by individuals and non-State actors. This appropriation is manifested in the practices of terrorists and radical organisations including Al-Qaeda and the ISIS, as well as by the resurgent Taliban in Afghanistan. In evaluating the adoption and implementation of the Jihad ideology by contemporary terrorist or radical organisations, this article assesses the future direction of this doctrine within contemporary Muslim societies. Third, Jihad, overtime, adopted the form of Islam's bellum iustum and enunciated rules justifying or legitimising the use of force as well as determining and regulating the conduct of hostilities. The Jihad ideology segments and distinguishes between what came to be subsequently termed as jus ad bellum (the rules justifying use of force) and jus in bello (rules regulating the conduct of hostilities). This study investigates the various substantial juristic dissensions that exist in the conceptualisation of the pre-modern Jihad principles relating to jus in bello and jus ad bellum. Finally, while, the Jihad ideology continues to eclipse the historiography of Islamic international law, this article investigates and establishes that siyar also became engaged with the important subjects of the legal status of recognised non-Muslim communities (known as dhimmis) within dar-al-Islam (the abode of Islam) as well as with issues pertaining to jurisdiction, commercial relations and diplomacy. I.